I am not from Japan, nor am I based in Japan, but my mother is Japanese and she goes back quite often, and I always ask her to be on the lookout for Tamagotchis for me.
My grandparents also always keep an eye open for Tamagotchi stuff to send to me.
My impression is, it's actually almost as hard to find Tamagotchi there as it is in the rest of the world. Tamagotchi are super super hot and ultra trendy, so when they first come out, people are lining up like crazy to get them, so they're sold out half the time. But when a new one comes out, the old ones are long gone, because the trend has turned over to the new one. So I really don't think you'll see a Devilgotchi or Santaclautchi in a regular retail environment. I mean, my mom goes to Japan at least once a year and always comes back empty-handed (instead she brings me lots of Hamtaro stuff, hee hee).
When the Entama first came out in Japan, my grandparents couldn't even buy one in a store, because they were all sold out all the time. They had to do a mail-in order from some reseller who was listed in a newspaper. They couldn't even choose the colour. (I got the orange one, which might not have been my personal choice, but I feel really special for getting one this way at all.) And that's the only time my "connections" in Japan have ever successfully acquired a Tamagotchi. Everything else has been from eBay, which seems comparably easier!
So as a tourist, I imagine it would be very very hard to locate a Tamagotchi source, unless something new happened to be coming out during your trip and you felt like spending a lot of your vacation lining up for the store to open (which I doubt).
Hopefully someone else here or elsewhere will have a good secret tip of a way to get Tamagotchi. (Of course if they put it on the Internet it won't be secret anymore.)